Investigation of Telomerase-dependent and Telomerase-independent Mechanisms of Telomere Homeostasis

Telomeres, the natural ends of linear eukaryotic chromosomes, are essential for cell viability and genome integrity. Telomeres are specialized DNA-protein complexes and function as protective caps to prevent chromosome ends from undergoing deleterious degradation and fusion events. Fission yeast shelterin, consisted of six-protein complex, exclusively associates with the telomeres. The shelterin is composed of telomeric sequence-specific double-stranded and single-stranded DNA binding partners, Rap1, Poz1, and Tpz1, which form a bridge connecting Taz1 and Pot1. Despite vital roles of the shelterin components in telomere length regulation, little is known about how they interact with each other and how these interactions contribute to telomerase regulation and telomere end protection. We demonstrate that Tpz1-mediated complete linkage within the shelterin, bridging telomeric dsDNA to ssDNA, controls the telomerase-nonextendible state. Moreover, Tpz1 participates in the activation of telomeres to the telomerase-extendible state via its interaction with Ccq1. Although Rap1’s role in telomerase regulation is rather well-understood, the functional roles of its evolutionarily conserved BRCT domain remain largely unknown. Here we find a novel interaction between Rap1BRCT domain and γH2A. This interaction in subtelomeric chromatin prevents activation of DNA damage response (DDR) by competitively inhibiting the interaction between γH2A and checkpoint mediator Crb253BP1. As telomeres undergo progressive shortening, gradual loss of Rap1-γH2A interaction allows Crb253BP1 to access subtelomeric γH2A, activate DDR and followed by trigger cellular senescence. Therefore, we propose that Rap1 functions as a telomere length-based licensing factor for cellular senescence